This invention relates generally to flowmeters of the vortex-shedding type and more particularly to a contoured shedding body for meters of this type which functions to minimize the pressure drop in the meter without impairing the operating efficiency thereof.
Under certain circumstances, the presence of an obstacle in a flow conduit will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed. These patterns are referred to as Karman vortex streets. The frequency at which vortices are shed in a Karman vortex street is a function of flow rate.
In this vortex-type flowmeter disclosed in Burgess U.S. Pat. No. 3,589,185, the obstacle assembly mounted in the flow conduit is constituted by a block positioned across the conduit with its longitudinal axis at right angles to the direction of fluid flow, a strip being supported behind a block and being spaced therefrom to define a gap which serves to trap Karman vortices and to strengthen and stabilize the vortex street. This vortex street is sensed to produce a signal whose frequency is proportional to flow rate.
In Herzl U.S. Pat. No. 3,867,839, the obstacle assembly disclosed therein also makes use of a block mounted across the flow conduit, but this block has a triangular cross-section with its apex being pointed toward the incoming fluid. Obstacle bodies having other cross-sectional shapes including a cylindrical form are disclosed in the Bird U.S. Pat. No. 3,116,639, and in the Rodley U.S. Pat. No. 3,572,117.
The advantage of a vortex-shedding body having a cylindrical form is that it is physically strong, mechanically stable and offers adequate internal space within which to mount various sensing systems. But these advantages are offset by the fact that a cylinder produces an irregular shedding action and does not have a very large operating range of constant meter coefficients, to say nothing of a number of other problems which militate against the use of a cylindrical shedding body.
It is known that flat plates afford the strongest shedding phenomenon. On the other hand, flat plates are physically weak and do not provide sufficient internal space for mounting sensing systems. For example, in the Herzl U.S. Pat. No. 3,946,608, the obstacle body has a trapezoidal cross-section, and it becomes possible with this shape to mechanically transmit the vibrations of a deflectable section cantilevered from the rear of this body to an external coupling point by means of a rod passing through an internal duct in the body. This is difficult to do with a flat plate.
The problems encountered with obstacle assemblies of the type heretofore known are aggravated in flowmeters of relatively small size. Thus only one manufacturing company has been able to produce a vortex-type flowmeter with a flow tube diameter of less than 2 inches, the meter including a flat plate obstacle. But the limitations imposed on the sensing system by this obstacle shape and the concomitant hydraulic problems are such that these meters have been marginal in operation.
In our copending application Ser. No. 758,489, above-identified, there is disclosed an obstacle assembly for a small diameter vortex-shedding flowmeter that is both mechanically and hydraulically efficient and whose internal volume is more than adequate to accommodate a sensing system whereby the meter is capable of shedding effectively at low Reynolds numbers and affords a meter linearity of better than .+-. 1%.
The assembly disclosed in our copending application includes a block positioned across the tube at right angles to the direction of fluid flow and a sensing vane cantilevered behind the block by means of a resilient beam, such that vortices shed by the block produce fluidic oscillations which cause the vane to vibrate at a frequency proportional to flow rate. The flat front face of the block is presented to the incoming fluid, the rear corners of the block being bevelled to define a flat rear face of reduced area. The resultant block configuration acts to enhance the shedding characteristics of the obstacle assembly and to produce substantially linear flow rate measurements at low Reynolds numbers.
In a flowmeter of the type disclosed in our copending application, the shedding characteristics are mainly determined by the width of the block; the wider the shedder, the lower the shedding frequency for a given flow rate. It has long been recognized that for satisfactory flowmeter performance, the ratio of the width of the shedding obstacle to the internal diameter of the flow tube must not be too small. But this relationship is not highly critical, and, as indicated in our copending application, an acceptable width may lie in the range of about 0.15 to 0.35 of the internal tube diameter. In practice, however, the low end of this range is usually about 0.20.
A vortex-shedding flowmeter interposed in a fluid line introduces a pressure drop therein which is approximately equal to the square of the width of the shedding body; hence the wider the obstacle, the greater the drop. As a consequence, an obstacle having a width large enough to afford good shedding characteristics usually gives rise to a substantial pressure drop. A large pressure drop has certain practical drawbacks; for to make up the pressure drop, pumping power is required, and this contributes to the installation and operating costs of the system. Moreover, the flowmeter will cavitate at high flow if the system pressure is low.
Yet with obstacle configurations of the type heretofore known, including the configuration disclosed in our copending application, though one can decrease the pressure drop of the meter merely by reducing the width of the obstacle, this reduction is at the expense of its shedding characteristics. Hence it has not heretofore been possible to effect a substantial reduction in pressure drop without adverse effects on meter performance.